This invention relates to a regenerative adsorption process and an adsorption chiller designed for utilising waste heat typically having a temperature of below about 150xc2x0 C. for useful cooling.
Two-reactor adsorption chillers have already been successfully commercialised in Japan [1,2]. By making use of a silica gel-water working pair, such chillers have managed to economically harness the potential of low-grade waste heat for useful cooling before it is discharged into the environment. Insofar as adsorption chillers are concerned, some methods have been devised to improve the conversion efficiency of the potential waste heat to useful cooling. For example, schemes have been proposed where such waste heat is used serially in a string of adsorption chillers before it is finally discharged. As another example, a scheme has previously been proposed where the desorption temperature is significantly reduced by means of multi-stage thermal compression of the refrigerant vapour [3]. This enables waste heat to be further utilized before it is finally purged to the environment. From the trend of development of the prior art, it would be desirable to further improve the conversion efficiency so that maximum cooling capacity can be derived from a given hardware investment, waste heat and coolant flow rate.
Of equal importance is the need for a stable chilled water outlet temperature. Based on experimental measurement on a commercially available 10 kW two-reactor adsorption chiller, under a typical dynamic steady state operation, the chilled water outlet temperature generally fluctuates by xc2x11.5xc2x0 C. [4]. While such fluctuation may be acceptable for sensible cooling and rough process cooling requirements, it begins to pose a problem in dehumidification, and other stringent cooling applications. In the latter field of usage, vapour compression or absorption cooling devices have been employed downstream to attenuate the temperature oscillation. It would therefore be desirable to provide a smoother chilled water outlet temperature so that downstream temperature smoothening devices could be downsized or even eliminated.
Sato et al. [5-6] have proposed a multi-reactor strategy involving cooling the adsorber with refrigerant emanating from one or more evaporators. It may be desirable and more practical to have the evaporator devoted to cooling the chilled water, with the evaporated refrigerant being superheated at the adsorbers. Master-and-slave configuration is commonly found in these references for the arrangement of the reactors. Such master-and-slave configurations for the string of reactors may represent an under-utilization of downstream reactors. It would therefore be attractive to eliminate such rigid configuration.
Many other designs [7-14] employ re-circulating fluid to boost the chiller""s coefficient of performance. These arrangements are designed for use with a high temperature heat source, which is usually economically valuable; they are done at the expense of a lower firing temperature for the desorber and a higher cooling temperature for the adsorber. In the case of low temperature (typically 150xc2x0 C. or below) waste heat application, such a strategy may not be feasible. In this case, the objective would then be to maximise the cooling throughput of the chiller.
The present invention advantageously improves the recovery efficiency of waste heat to useful cooling. Recognizing that cooling water for the adsorber and condenser is a scarce resource, the invention aspires to achieve maximum cooling capacity for a given flow rate of waste heat and cooling stream. This advantageously also ensures maximum conversion efficiency of waste heat to useful cooling and reduces piping material for a given cooling capacity.
Advantageously, the invention also makes it possible to downsize or even eliminate the need for downstream temperature smoothening devices by providing a more stable chilled water outlet temperature.
Further, the invention advantageously reduces the risk of ice formation by providing for a sequential start-up of the reactor or reactors when the chiller is activated.
According to one aspect of the invention there is provided a regenerative adsorption process for application in an adsorption assembly comprising a condenser, an evaporator and a plurality of reactors each alternately operating in adsorption and desorption modes, said process comprising:
passing a coolant through the condenser;
passing the coolant emanating from the condenser through reactors operating in adsorption mode; and
passing waste heat from a waste heat source through reactors operating in desorption mode; wherein said plurality of reactors are scheduled such that each reactor alternately operates in adsorption and desorption modes for substantially identical time intervals, and such that each reactor has an equal chance of being the first reactor to receive the coolant emanating from the condenser when operating in adsorption mode, and the waste heat from the waste heat source when operating in desorption mode.
According to another aspect of the invention there is provided a multi-reactor regenerative adsorption chiller assembly comprising:
a condenser adapted to receive a coolant from a source;
an evaporator connected to said condenser to provide a refrigerant circuit;
a plurality of reactors, each being able to operate in adsorption and desorption modes and having a coolant inlet to directly or indirectly receive coolant emitted from said condenser when operating in adsorption mode, and a waste heat inlet for directly or indirectly receiving waste heat from a waste heat source when operating in desorption mode; and control means for controlling said plurality of reactors such that each reactor alternately operates in adsorption and desorption modes for substantially identical time intervals, and such that each reactor has an equal chance of being the first reactor to receive the coolant emanating from the condenser when operating in adsorption mode, and the waste heat from the waste heat source when operating in desorption mode.
The reactors operating in adsorption mode may be arranged in series and/or in parallel depending upon the particular operation, and also depending on the total number of reactors being used. However, the reactors operating in desorption mode are arranged in series.
In a preferred embodiment, the plurality of reactors comprises an even number of reactors, wherein at substantially any instant during the process, half of the plurality of reactors operate in adsorption mode and the other half of the plurality of reactors operate in desorption mode. Most preferably, the plurality of reactors comprises at least four reactors.
The flow rate of coolant and waste heat through the plurality of reactors operating in adsorption and desorption modes respectively maybe any suitable flow rate depending on the particular size of chiller assembly and design of heat exchangers. Preferably, the coolant is flowed through the reactors operating in adsorption mode at a suitable flow rate. A suitable flow rate is preferably any flow rate that result in a transition or turbulent flow regime in the channel of a heat exchanger, be it the chilled water, coolant and/or heat source. When the plurality of reactors comprises four or more reactors, the flow rate of coolant through reactors operating in adsorption mode is preferably at the suitable flow rate, irrespective of whether the reactors operating in adsorption mode are arranged in series or in parallel. Sizing and flow rates can be determined by those who are skilled in the art.
The waste heat is preferably flowed through the reactors operating in desorption mode at a suitable flow rate. More preferably, where the plurality of reactors comprises four or more reactors, the flow rate of waste heat through reactors operating in desorption mode is also sized at the suitable flow rate.
Similarly, the flow rate of coolant through the condenser may be determined for a specific application of the invention. It will be recognised that the flow rate of coolant through the reactors operating in adsorption mode will be somewhat dependent on the flow rate of coolant through the condenser. In a preferred embodiment, the flow rate of coolant through the condenser is at a suitable flow rate as described above.
As discussed above, the adsorption assembly comprises a condenser, an evaporator and a plurality of reactors, each of which alternatively operates in adsorption and desorption modes. In a preferred embodiment, the plurality of reactors are arranged in series such that, in use, reactors operating in adsorption mode constitute a first sub-series of reactors connected in series and/or in parallel to receive coolant from the condenser and reactors operating in desorption mode constitute a second sub-series of reactors connected in series to receive waste heat from the waste heat source.
Each reactor is preferably composed of heat exchanging material and contains adsorbents. The adsorbent could be any material, such as silica gel, that is able to adsorb, either by physisorption and/or chemisorption refrigerant, for example water vapour, ammonia, or methanol at a typical cooling tower temperature and desorb refrigerant at moderately low temperature (typically 150xc2x0 C. or below). The coolant from a cooling tower is first passed through the condenser and subsequently to each of the reactors operating in adsorption mode either in series or in parallel. The waste heat source is passed serially from one reactor operating in desorption mode to another reactor in the same mode. After passing through the last reactor operating in desorption mode, the waste heat is purged from the system.
The reactors are scheduled such that each reactor alternately operates in adsorption and desorption mode for substantially the same time interval, and that each reactor has equal chance of being the first reactor to either receive the coolant emanating from the condenser or the waste heat. Such a schedule ensures that maximum smoothening of chilled water outlet temperature is achieved. This arrangement also facilitates maximum extraction of energy from the waste heat to maximise cooling capacity. Cooling the condenser first and then the reactors operating in adsorption mode ensures that minimum coolant flow rate is used.